Structure and Electronic Configurations of the Intermediates of Water Oxidation in Blue Ruthenium Dimer Catalysis

Abstract

Catalytic O(2) evolution with cis,cis-[(bpy)(2)(H(2)O)Ru(III)ORu(III)(OH(2))(bpy)(2)](4+) (bpy is 2,2-bipyridine), the so-called blue dimer, the first designed water oxidation catalyst, was monitored by UV-vis, EPR, and X-ray absorption spectroscopy (XAS) with ms time resolution. Two processes were identified, one of which occurs on a time scale of 100 ms to a few seconds and results in oxidation of the catalyst with the formation of an intermediate, here termed [3,4]'. A slower process occurring on the time scale of minutes results in the decay of this intermediate and O(2) evolution. Spectroscopic data suggest that within the fast process there is a short-lived transient intermediate, which is a precursor of [3,4]'. When excess oxidant was used, a highly oxidized form of the blue dimer [4,5] was spectroscopically resolved within the time frame of the fast process. Its structure and electronic state were confirmed by EPR and XAS. As reported earlier, the [3,4]' intermediate likely results from reaction of [4,5] with water. While it is generated under strongly oxidizing conditions, it does not display oxidation of the Ru centers past [3,4] according to EPR and XAS. EXAFS analysis demonstrates a considerably modified ligand environment in [3,4]'. Raman measurements confirmed the presence of the O-O fragment by detecting a new vibration band in [3,4]' that undergoes a 46 cm(-1) shift to lower energy upon (16)O/(18)O exchange. Under the conditions of the experiment at pH 1, the [3,4]' intermediate is the catalytic steady state form of the blue dimer catalyst, suggesting that its oxidation is the rate-limiting step.

Experimental demonstration of radicaloid character in a RuV=O intermediate in catalytic water oxidation

“…2C. This signal has been observed earlier (21,37) and was assigned to a signal from the S = 1/2 spin state with g xx = 2.03, g yy = 1.98, g zz = 1.87 g-tensor; g xx = 2.03 and g zz = 1.87 are clearly resolved in the spectrum, whereas g yy = 1.98 was derived from spectral simulations ( Fig. 2C and Fig.…”

“…Based on earlier literature reports (38,40), this shift is in good agreement with assignment of the 816-cm −1 band to a Ru V = O vibration. The 816-cm −1 band grows in relative intensity from 100 ms to 4 s and then decreases after 7 s, giving rise to a band at 683-688 cm −1 previously assigned to a BD [3,4]-prime species (37). At the same time, the band for the Ru-O-Ru bridge mode shifts from 390 (at 4 s) to 410 cm −1 (at 15 s) (37).…”

“…The BD catalyst intermediates BD [4,5] and BD [5,5], which are reactive to water, were characterized in previous studies (29,(35)(36)(37) and are characterized in more detail in this work. BD [4,5] can be prepared in 0.1 M HNO 3 by oxidation of the BD [3,4] with excess of cerium ammonium nitrate [Ce(IV)] and fast (less than 10-15 s) freezing (37).…”

“…BD [4,5] can be prepared in 0.1 M HNO 3 by oxidation of the BD [3,4] with excess of cerium ammonium nitrate [Ce(IV)] and fast (less than 10-15 s) freezing (37). The BD [4,5] intermediate has been characterized by Ru K-edge X-Ray Absorption Near Edge Structure (XANES) (Fig.…”

Behavior of the Ru-bda Water Oxidation Catalyst Covalently Anchored on Glassy Carbon Electrodes

“…Finally, molecular WOCs also benefit from an arsenal of spectroscopic techniques that can be applied to molecules and allow to derive detailed information on molecular and electronic structures. 17 In addition, anchoring WOCs on electrode surfaces is a very attractive strategy for generation of hybrid materials for heterogeneous water oxidation. 18,19,20,21,22,23,24 Hybrid materials are very attractive since they can provide a large degree of flexibility to build photoelectrochemical cells for water splitting.…”